1. Field of the Invention
This invention relates generally to spin valve magnetic transducers for reading information signals from a magnetic medium and, in particular, to an improved antiparallel-pinned spin valve sensor, and to magnetic recording systems which incorporate such sensors.
2. Description of Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an "MR element") as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flow through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material separated by a layer of non-magnetic electrically conductive material are generally referred to as spin valve (SV) sensors manifesting the SV effect. In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fe--Mn) layer. The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). In SV sensors, the SV effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a as corresponding change in the sensed current or voltage.
FIG. 1 shows a prior art SV sensor 100 comprising end regions 104 and 106 separated by a central region 102. A free layer (free ferromagnetic layer) 110 is separated from a pinned layer (pinned ferromagnetic layer) 120 by a non-magnetic, electrically-conducting spacer 115. The magnetization of the pinned layer 120 is fixed by an antiferromagnetic (AFM) layer 121. Free layer 110, spacer 115, pinned layer 120 and the AFM layer 121 are all formed in the central region 102. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed over hard bias layers 130 and 135, respectively, provide electrical connections for the flow of the sensing current Is from a current source 160 to the MR sensor 100. Sensing means 170 connected to leads 140 and 145 sense the change in the resistance due to changes induced in the free layer 110 by the external magnetic field (e.g., field generated by a data bit stored on a disk).
IBM's U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses an MR sensor operating on the basis of the SV effect.
Another type of spin valve sensor currently under development is an antiparallel (AP)-pinned spin valve sensor. FIG. 2 shows an AP-Pinned SV sensor disclosed in copending Application Ser. No. 08/697,396 by Fontana et al., filed Aug. 23, 1996, and assigned to the assignee of the present invention. In the AP-pinned SV sensor 200 of FIG. 2, the pinned layer is a laminated structure of two ferromagnetic layers separated by a non-magnetic coupling layer such that the magnetizations of the two ferromagnetic layers are strongly coupled together antiferromagnetically in an antiparallel orientation. The exchange coupling between the antiferromagnetic (AFM) layer and the laminated pinned layer of AP-pinned SV sensor of FIG. 2 is substantially stronger than the exchange coupling between the AFM layer and the single pinned layer of the SV sensor of FIG. 1. This improved exchange coupling increases the stability of the AP-pinned SV sensor at high temperatures which allows the use of corrosion resistant antiferromagnetic materials such as NiO for the AFM layer.
Referring again to FIG. 2, a free layer 210 is separated from a laminated AP-pinned layer 220 by a nonmagnetic, electrically-conducting spacer layer 215. The magnetization of the laminated AP-pinned layer 220 is fixed by an AFM layer 230 which is made of NiO. The laminated AP-pinned layer 220 includes a first ferromagnetic layer 222 (PF1) and a second ferromagnetic layer 226 (PF2) separated from each other by an antiparallel coupling (APC) layer 224 of nonmagnetic material. The two ferromagnetic layers 222, 226 in the laminated AP-pinned layer 220 are formed of Co and the APC layer 224 is formed of Ru. The AFM layer 230 is formed on a seed layer 240 deposited on the substrate 250. To complete the AP-pinned SV sensor, a capping layer 205 is formed on the free layer 210.
A key advantage of the AP-pinned SV sensor of FIG. 2, that allows the use of Nio material for the AFM layer, is the improvement of the exchange coupling field strength between the AFM layer 230 and AP-pinned layer 220. This exchange coupling field is inversely proportional to the magnetic moment difference (net magnetic moment) between the two AP-pinned ferromagnetic layers.
However, experiments by the present inventor on fabrication of AP-pinned SV sensors of FIG. 2 (layered structure of NiO/Co/Ru/Co/Cu/Ni--Fe/Ta) have shown that the net moment of the laminated AP-pinned structure of Co/APC/Co is very difficult to control and reproduce because of interfacial diffusion and oxidation that takes place at the interface between the NiO AFM layer and the first ferromagnetic layer of Co. The interfacial diffusion and oxidation that takes place at the aforementioned interface causes a change in the moment of the first ferromagnetic Co layer even after the AP-pinned SV sensor of FIG. 2 has been completely built. The change in the moment of the first ferromagnetic layer causes the change in the net moment of the AP-pinned layer. These experiments have resulted in variations by factors of 2 to 3 in the net moments of the AP-pinned layers for successive fabrication runs. Variations in net moment of the AP-pinned structure results in large variations in pinning fields which compromises the stability of the SV sensors. Furthermore, experiments conducted have shown that Co deposited on an NiO AFM layer has a very large coercivity, but low exchange pinning to the NiO AFM layer. The large coercivity of the Co layer makes it very difficult to reset the magnetization direction of the pinned layer if such a reset becomes necessary. Such a reset may become necessary if the magnetization direction of the pinned layer becomes disoriented, for example, in the disk drive due to a large unexpected magnetic field.
Therefore, there is a need for an AP-pinned SV sensor where the AP-pinned layer has a well controlled and reproducible net moment.